CAVEAT: IANAL, so consult one, as always.
this is an excellent question, and it is the reason for collaboration agreements signed.
With a CA, you can avoid all the pitfalls of the default empowerments granted by copyright law on collaborations
and the operative word, if you read the law, and you should,
it says that a "copyright exists when the work is in tangible form".
However, things are a bit different with derivative works.
If you don't have a CA signed, make sure you don't record
the final result of both lyric and music, or put it to
a sheet music because ( i.e., "tangible form" )
that is the point at which all the
empowerments granted copyright to collaborators kick in
( which is why I have CA's signed, so I can record without it being
a "copyright" because I get the collaborator to agree
in writing this is so, i.e, that the "tangible form" does not
exist as a state of copyright, until I, as composer, register the copyright via my online account at www.copyright.gov
and I, as composer, reserve the right to execute the registration ).
Get an attorney to write the collaboration agreement to your specifications, and consult with an attorney, because, yes,
there are serious ramifications you need to know before you
embark on any collaboration with anyone. if whomever you collaborate doesn't want to sign a CA, if
you think you are really talented and might some day make it big, well, I wouldn't work with that person, if I were you, but that
is me. and, the big reason is for bad experiences I have and later regretted that I didn't have a CA signed.
Pat Hardy Lockwood